Manganese-based nanoscale metal-organic frameworks for magnetic resonance imaging

Article abstract of DOI:10.1021/ja803777x

Manganese-containing nanoscale metal-organic frameworks (NMOFs) with controllable morphologies were synthesized using reverse-phase microemulsion techniques at room temperature and a surfactant-assisted procedure at 120 °C with microwave heating. The nanoparticles were characterized using a variety of methods including SEM, TEM, TGA, PXRD, and ICP-MS. Although the nanoparticles gave a modest longitudinal relaxivity (r₁) on a per Mn basis, they provided an efficient vehicle for the delivery of large doses of Mn²⁺ ions which exhibited very high in vitro and in vivo r₁ values and afforded excellent MR contrast enhancement. The particle surface was also modified with a silica shell to allow covalent attachment of a cyclic RGD peptide and an organic fluorophore. The cell-targeting molecules on the Mn NMOFs enhanced their delivery to cancer cells to allow for target-specific MR imaging in vitro. The MR contrast enhancement was also demonstrated in vivo using a mouse model. Such core-shell hybrid nanostructures provide an ideal platform for targeted delivery of other imaging and therapeutic agents to diseased tissues. Copyright

Full text of DOI:10.1021/ja803777x

Published on Web 10/10/2008
Manganese-Based Nanoscale Metal-Organic Frameworks for Magnetic
Resonance Imaging
Kathryn M. L. Taylor, William J. Rieter, and Wenbin Lin*
Department of Chemistry, CB#3290, UniVersity of North Carolina, Chapel Hill, North Carolina 27599
Received May 20, 2008; E-mail: wlin@unc.edu
Magnetic resonance imaging (MRI) is a powerful noninvasive
diagnostic tool that provides high spatial resolution images and does
not involve radioactivity.1 Intrinsically low sensitivity of MRI
however necessitates the use of contrast agents that are often
administered in high doses.2 Paramagnetic and superparamagnetic
nanomaterials have recently emerged as promising MR contrast
agents owing to their ability to carry large payloads of magnetic
centers.3 They can work at very low concentrations and be made
target-specific by conjugation with affinity molecules.4
Figure 1. SEM (a) and TEM (b) images of Mn(BDC)(H2O)2 nanorods (1).
We recently demonstrated the synthesis of Gd-based nanoscale
metal-organic frameworks (NMOFs) with extraordinarily high MR
relaxivities on a per particle basis. The toxicity of leached Gd3+ ions
however precludes clinical applications of these NMOFs. Given the
modular nature of MOFs, their compositions and structures can be
readily tuned by a judicious choice of metal ions and organic linkers
to lead to MR-enhancing NMOFs that are more biocompatible and
less toxic. Indeed, Mn2+ centers are much less toxic than Gd3+ centers
and have been shown to exhibit very high in vivo longitudinal (r1)
MR relaxivities by binding to intracellular proteins.5 Herein we wish
to report the synthesis of Mn NMOFs, their coating with a thin silica
shell,andsubsequentfunctionalizationwithacyclicarginine-glycine-aspartate
(RGD) peptide and a fluorophore, and the application of such
core-shell nanostructures in MR imaging.
Mn NMOFs with terephthalic acid (BDC) and trimesic acid
(BTC) bridging ligands were synthesized in reverse-phase micro-
emulsions.6 Nanorods of Mn(BDC)(H2O)2 (1) were synthesized by
stirring a CTAB/1-hexanol/n-heptane/water microemulsion with a
Figure 2. SEM (a) and TEM (b) images of Mn3(BTC)2(H2O)6 spiral
W-value of
5 that contained equal molar MnCl2 and
nanorods (2) synthesized at room temp; (c) SEM image of nanoparticles of
2 synthesized at 120 °C under microwave heating; (d) TEM image of silica-
coated nanoparticles of 2 that were under microwave heating.
[NMeH3]2(BDC) for 18 h at room temp. SEM and TEM micro-
graphs show that particles of 1 adopt a rodlike morphology with
diameters of 50-100 nm and lengths of 750 nm to several µm
(Figure 1). Powder X-ray diffraction (PXRD) studies indicated that
nanorods of 1 are crystalline and correspond to the known bulk
phase of Mn(BDC)(H2O)2.7 The composition was also confirmed
by TGA and ICP-MS results.
Nanoparticles of Mn3(BTC)2(H2O)6, 2, were similarly prepared
in a CTAB/1-hexanol/isooctane/water microemulsion (W ) 10) with
a Na3(BTC)/MnCl2 molar ratio of 2:3. SEM and TEM micrographs
show that particles of 2 are fairly uniform and adopt an unusual
spiral rod morphology with diameters of 50-100 nm and lengths
of 1-2 µm (Figure 2a,b).8 The spiral nanorods are crystalline based
on PXRD but do not correspond to any known phase of Mn-BTC
MOFs.9 The composition of 2 was established on the basis of TGA
and ICP-MS results.
We also carried out surfactant-assisted synthesis of 1 and 2 under
microwave heating in an attempt to alter their morphologies. Nanorods
of 1 were obtained after heating a CTAB/1-hexanol/n-heptane/water
microemulsion of MnCl2 and [NMeH3]2(BDC) in 1:1 molar ratio with
a W-value of 10 at 120 °C for 10 min in an 800 W microwave. SEM,
PXRD, and TGA studies showed that nanorods of 1 obtained under
microwave heating have identical phase and similar morphology to
those obtained from the room temperature reaction. Interestingly,
microwave heating of a CTAB/1-hexanol/isooctane/water microemul-
sion of Na3(BTC) and MnCl2 (in 2:3 molar ratio) with a W-value of
10 at 120 °C for 10 min produced nanoparticles of 2 with a much
lower aspect ratio. PXRD and TGA indicated that nanoparticles of 2
obtained under microwave heating are the same phase as those obtained
at room temperature, but SEM and TEM images showed that the
particles obtained under microwave heating have a blocklike morphol-
ogy, with lengths ranging from ∼50 to 300 nm in the three dimensions
(Figure 2c).
Microwave-synthesized nanoparticles of 2 were coated with a thin
silica shell (denoted as 2′) to stabilize them and to facilitate their
functionalization with a fluorophore and a cell-targeting peptide.
Nanoparticles of 2′ were prepared by base-catalyzed condensation of
TEOS on PVP-modified particles of 2 in ethanol (Scheme 1).10 TEM
images indicated the presence of a thin shell of amorphous silica on
2′ (Figure 2d), which was confirmed by TGA since 2′ had an additional
7.1% weight remaining after heating at 600 °C in air. Rhodamine B
and c(RGDfK) were grafted onto 2′ by treatment with RITC-APS (1.5
wt %) and tri(ethoxy)silylpropyl carbomoyl c(RGDfK) (∼10 wt %)
in a basic ethanolic suspension. Rhodamine B provides characteristic
fluorescence for optical imaging whereas c(RGDfK) is a small cyclic
9
14358 J. AM. CHEM. SOC. 2008, 130, 14358–14359
10.1021/ja803777x CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
Scheme 1
peptide that can target angiogenic cancers (such as HT-29) by binding
to the upregulated Rvꢀ3 integrin.
We determined MR relaxivities of 1 and 2 on a 3 T scanner.
Nanorods of 1 were found to have a longitudinal relaxivity (r1) of 5.5
and a transverse relaxivity (r2) of 80.0 mM-1 s-1 on a per Mn basis,
whereas nanorods of 2 exhibited an r1 of 7.8 and an r2 of 70.8 mM-1
s-1 on a per Mn basis. Consistent with this, nanorods of 2 had an r1
of 4.6 and an r2 of 141.2 mM-1 s-1 on a per Mn basis at 9.4 T. 2′ has
slightly lower r1 of 4.0 and r2 of 112.8 mM-1 s-1 at 9.4 T. The slight
decrease of MR relaxivities is expected because of the reduced
influence of the Mn centers on the surrounding water molecules.
The r1 values exhibited by the Mn NMOFs are modest and these
nanoparticles are not expected to be efficient T1-weighted contrast
agents. Given the typically high water solubility of most MOFs,
we hypothesized that the NMOFs might provide an efficient vehicle
for the delivery of large doses of Mn2+ ions which are known to
exhibit very high r1 values inside cells.5 Leaching studies indicated
that 2′ has a half-life of 7.5 h (i.e., about ∼50% Mn2+ ions were
released from 2′) whereas uncoated 2 has a half-life of 3.5 h in
water at 37 °C (Figure 3a). In PBS buffer at 37 °C, the half-lives
for 2 and 2′ were reduced to 18 min and 1.44 h, respectively. These
results indicate the stabilization of the Mn NMOFs by silica coatings
and suggest that the silica-coated particles should have adequate
time to reach the site of interest where they will then release Mn2+
to give T1-weighted contrast enhancement.
Figure 3. (a) Dissolution curves of uncoated (blue) and silica-coated (red)
Mn3(BTC)2(H2O)6 nanoparticles (2 and 2′) in water at 37 °C (% released
vs time). (b) In vitro MR images of HT-29 cells incubated with no 2′ (left),
nontargeted 2′ (middle), and c(RGDfK)-targeted 2′ (right). (c-e) Merged
confocal images of HT-29 cells that were incubated with no 2′ (c),
nontargeted 2′ (d), c(RGDfK)-targeted 2′ (e). The blue color was from
DRAQ5 used to stain the cell nuclei while the green color was from
rhodamine B. The bars represent 20 µm.
enhancement. Surface functionalization of the Mn NMOFs with a cell-
targeting molecule enhances their delivery to cancer cells to allow for
target-specific MR imaging. Such a core-shell nanostructure platform
can be used for targeted delivery of other imaging and therapeutic
agents.
Acknowledgment. We acknowledge financial support from NIH
(Grant U54-CA119343) and NSF. W.J.R. thanks NSF for a graduate
fellowship. We thank Dr. Jason S. Kim, Dr. Hongyu An, and Mr.
Joe Della Rocca for experimental help.
Supporting Information Available: Experimental procedures and
30 figures. This material is available free of charge via the Internet at
http://pubs.acs.org.
We evaluated the efficacy of 2′ that have been surface-functionalized
with rhodamine B and c(RGDfK) as MR and optical contrast agents
in vitro. Human colon cancer (HT-29) cells were incubated with
rhodamine B-functionalized nanoparticles of 2′ with and without the
c(RGDfK) targeting peptide. In vitro MR imaging of HT-29 cells
showed the selective uptake of 2′ and thus the intracellular delivery
of Mn2+ ions. As shown in Figure 3b, the HT-29 cells incubated with
the targeted particles gave much higher signals in T1-weighted images
than those that were not incubated with nanoparticles as well as those
that were incubated with the nontargeted particles. ICP-MS analysis
supported the enhanced uptake of 2′ by HT-29 cells: the cell pellets
(∼3.2 million cells) contained 0.05, 0.33, and 2.29 µg of Mn after
incubating with no particles, nontargeted 2′, and targeted 2′, respec-
tively. Confocal microscopic imaging studies further confirmed the
enhanced uptake of the particles with the targeting peptide, compared
to those without the targeting peptide (Figure 3c,d). These results
demonstrate the target-specific uptake of the c(RGDfK)-modified Mn
NMOFs by angiogenic cancer cells, presumably via receptor-mediated
endocytosis. Finally, we also demonstrated the in vivo utility of Mn
NMOFs for Mn2+ ion delivery. T1-weighted contrast enhancement
was observed in the mouse liver, kidneys, and aorta ∼1 h after tail
vein injection of 2′ at a 10 µmol/kg Mn dose (Supporting Information),
apparently caused by the Mn2+ ions released from the nanoparticles.
In summary, we have synthesized Mn NMOFs with controllable
morphologies and demonstrated their potential for MR contrast
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